Data encryption is well known and has been in use for a number of years. A problem arises in packet switching networks in that whereas it is possible to encrypt the data contained within the packet, the packet itself, as it is transmitted between packet switching networks via interconnecting trunks, is defined by flag and header information. Hence, the encryption of the data is not as effective since the amount of data being encrypted is relatively small, and the probability of the encryption algorithm being broken is greatly increased. Indeed, the packet header information contains the logical channel number which allows the identification of which packets are associated with particular frames being transmitted between packet switching systems.
One such prior art packet switching system is disclosed in U.S. Pat. No. 4,494,230, and is illustrated in FIG. 1. FIG. 2 illustrates the packet which would be transmitted in the packet switching system of FIG. 1 between customer 100 and customer 110. The data portion of the packet illustrated in FIG. 2 would be encrypted by customer 100 before transmission into the packet switching system, but the system adds flag information and the header information that consists of packet identifier, logical address, and time stamp fields. The packets in the above-cited U.S. patent are communicated via high-speed digital links across country via trunks such as trunk 118. These trunks could be microwave or satellite links. In the event that unauthorized personnel should gain access to trunk 118, the packets being communicated on trunk 118 would have the format illustrated in FIG. 2. Since the customer is only able to encyrpt the data portion of the packet of FIG. 2, the unauthorized personnel could readily break the encryption algorithm.
From the foregoing, it can be seen that there exists the need for a method of not only encrypting the data within a packet, but for encrypting the entire transmission path over which the packets flow, thus also encrypting the flag and header information. Whereas the present public key encryption systems perform extremely well in absence of errors, these systems require a large amount of processing, and more efficient and less intense processing algorithms are desirable. In the presence of errors, packets are likely to be lost during periods of re-synchronization using a public key encryption system.